Today's semiconductor devices are fabricated from a wafer of semiconductor material, such as silicon or the like, which is typically subjected to an epitaxial growth process and a diffusion process to establish at least one circuit on the wafer. Following such processing, the wafer is diced into individual chips which each contain a separate one of the circuits formed on the wafer. Prior to dicing the wafer, the wafer is commonly "probed" (i.e., electrically contacted) with a wafer probe especially designed for this task. The purpose in probing the wafer is to inject test signals into, and recover response signals from, each circuit on the wafer to verify its operation. Circuits which are found to be defective are marked so that after the wafer is diced, the defective circuits can be separated and discarded.
There are presently two main types of wafer probes in use for probing semiconductor wafers during their manufacture. The first type of probe consists of a plurality of rigid wires, typically made from tungsten or the like, each wire having one of its ends rigidly attached to a support member so as to be cantilevered therefrom. At the opposite end of each wire is a hardened tip which is adapted to make an electrical contact with a metallized area on the wafer to be probed. By moving the support member, the tips of the wires can be brought into contact with successive sets of metallized areas on the wafer to make contact with such areas for testing purposes.
The advantage of this type of probe is that the hardness of the tips of the wires allows them to "scrub" (i.e., scratch) the metallized areas on the wafer when the wires are moved with respect to the wafer or vice versa. The metallized areas, particularly when fabricated from aluminum, are often covered with an oxide layer which reduces their conductivity and thus impairs testing of the wafer. By displacing the wafer during probing relative to the probe (or vice versa), the tips of the wires will scrub the metallized areas and thereby break through whatever oxide layer is present to assure a reliable electrical connection with the wafer.
While this type of probe offers the ability to scrub the metallized areas on the wafer to assure a reliable electrical contact therewith, the probe is not without its disadvantages. One major disadvantage is that the individual signal-carrying wires are not shielded in any way thus, impairing the ability of the probe to test the wafer at high frequencies because of the increased likelihood of coupling and cross talk.
The other type of probe commonly in use generally consists of a plurality of strip line sections, each secured at one of its ends to a support. Each strip line section has a ground plane on its lower major surface which shields a signal line on the strip line section that terminates at a depending conductive bump (usually fabricated from a hard metal alloy) at the end of the strip line section opposite the one attached to the support member. The depending bump at the end of each strip line section serves as the means for making contact with a corresponding metallized area on the semiconductor wafer to be probed.
The advantage of the strip line-type fixture is that each of its ground planes shields each signal-carrying line passing between the wafer and the probe, thereby reducing the incidence of cross talk and coupling as compared to the wire-type probe described earlier. However, as compared to the wire-type probe, the strip line-type probe generally cannot scrub the metallized areas on the wafer because the conductive bump on each strip line section is not sufficiently rigid so that when the wafer is displaced relative to the probe, the bump could be detached from the line.
Thus, there is a need for a wafer probe which can scrub the metallized areas on a substrate, such as a semiconductor wafer, to make a good electrical contact therewith, while offering reduced incidence of cross talk and coupling between signals transmitted to and recovered from the substrate.